This invention relates generally to glass-ceramic cooktop appliances and particularly to electronic power control systems for such appliances which provide protection against overheating the glass-ceramic cooktop.
Use of glass-ceramic plates as cooktops is well known. Advantages of the smooth surface include pleasing appearance and easy cleanability. In conventional glass-ceramic cooktop appliances the glass-ceramic plate is heated by radiation from a heating unit such as an open coil electric resistive heater or gas burner disposed beneath the glass. The glass-ceramic is heated sufficiently to heat utensils upon it primarily by conduction from the heated glass-ceramic to the utensil. Due to the high thermal impedance of the glass-ceramic plate, such cooktops are less thermally efficient than conventional cooktops employing sheathed heating elements.
It has been found that the thermal efficiency of the glass-ceramic cooktop appliances can be significantly improved by using a heating unit which radiates substantially in the infrared region (1-3 microns) in combination with a glass-ceramic material which is transparent to such radiation. In such appliances a utensil placed on the cooktop is heated primarily by radiation transmitted directly from the heating unit to the utensil rather than by conduction from the glass-ceramic material. In addition to improved efficiency such appliances have the further advantage of responding more quickly to changes in the power level applied to the heating unit.
In both conventional glass-ceramic cooktops and radiant glass-ceramic cooktops, provision must be made to avoid overheating the cooktop. For most glass-ceramic materials the operating temperature should not exceed 700.degree. C. for any prolonged period. Under normal operating conditions the temperature of the glass-ceramic will remain below this limit. However, conditions can occur which can cause this temperature limit to be exceeded. Commonly occurring examples include operating the heating unit with no load, i.e. no utensil, on the cooktop surface, using badly warped utensils which make uneven contact with the cooktop surface and operating the heating unit with an empty utensil.
In conventional glass-ceramic cooktop appliances thermal protection is typically provided by simply turning off the heating unit when the temperature exceeds a predetermined temperature limit. Examples of such temperature limiter arrangements can be found in commonly assigned U.S. Pat. Nos. 3,885,128 and 4,237,368. In the '128 patent a sensor is provided which responds to the temperature of a heat spreader disposed between the glass-ceramic and the heater. This sensor turns off power to the heating unit when the temperature of the heat spreader reaches 1250.degree. F. In the '368 patent the temperature sensor utilizes the temperature resistance characteristic of the glass-ceramic plate as a means of monitoring temperature of the glass. As in the '128 patent power is simply removed from the heating unit when the sensed glass-ceramic temperature exceeds a predetermined temperature limit.
Since conventional glass-ceramic cooktops rely primarily on conduction from the glass-ceramic material to heat the load, the thermal inertia of the glass-ceramic is such that cooking performance is not significantly adversely affected by temporary interruptions in power to the heating unit. However, in cooktops which rely primarily on radiant energy from the heating unit to directly heat the utensil, shutting off power to the heating unit when the glass-ceramic material gets too hot and turning it on again when the temperature drops below a predetermined temperature presents a number of problems. For example, since the utensil is primarily heated by radiant energy from the heating unit, when the unit is turned off, the heating of the utensil drops off dramatically while waiting for the glass-ceramic to cool down. This adverse affect on cooking performance is heightened by the temperature hysteresis loop required in such temperature limiting schemes. Also, when the glass-ceramic overheats and power is removed, the heating unit cools down much more quickly than does the glass. Since radiant heating units are designed to be most efficient when operating at the highest possible temperature, the heating unit undergoes extreme temperature swings from near room temperature to operating temperature adversely affecting heating unit reliability and longevity.
In light of the foregoing limitations of the prior art there is a need for a means of limiting the temperature of the glass-ceramic plate in a glass-ceramic cooktop appliance which satisfactorily protects the glass-ceramic from overheating while minimizing any adverse affect on cooking performance and heating unit longevity.
It is therefore a primary object of the present invention to provide an improved power control system for a glass-ceramic cooktop appliance which protects the glass-ceramic from overheating without significantly adversely affecting cooking performance or heating unit longevity.